Surfboards, windsurfers, and small sailboats are water riding boards, or crafts, which generally include a foam core (unfinished board) in which a fin box is installed which holds a downwardly-projecting fin that provides directionality to the craft or board in water. A shallow groove is routed in the bottom surface of the board near its rear end, and the fin box is installed in the shallow groove, with the lower surface of the fin box even with the lower surface of the foam core. The sides and upper surface of the fin box may be bonded, as with adhesive, to the walls of the groove. The board is finished by "glassing" it, which involves applying a strong but thin surface layer to the entire surface of the foam core, such as a resin-impregnated fibrous layer. The resin also bonds to the bottom surface of the fin box.
It is very common for the fin boxes to break out of the board. For example, when the rear of a rapidly moving surfboard rises out of the water and is turned sidewardly before it again hits the water, the large sideward force applied to the fin may break the fin box out of the board. The large torque applied to the fin box is resisted by the "glass" surface layer and by the foam in the board, but the resistance is not sufficient to avoid frequent breakout of the fin box. A technique for securing the fin box against breakout, by the more effective transfer of torque applied to the fin box to the rest of the board, and which was of relatively low weight and low cost, would be of considerable value.